Effective diabetes self-management is central to maintaining euglycemia and reducing the morbidities associated with type 1 diabetes mellitus (T1D). Adherence to the complex regimen required to achieve ideal hemoglobin A1C (HbA1C) is commonly sub-optimal in adolescents with T1D. Thus, there is a critical need to establish the efficacy of a novel approach to delivery of diabetes self-management support that integrates key elements of proven strategies, that is tailored to address the ever-changing needs and opportunities for individual youth living with T1D, and that is feasible and immediately applicable to clinical settings. The specific objective o the proposed research, and the next logical step toward our goal, is to execute a full-scale randomized controlled trial (RCT) of the Flexible Lifestyles (FL3X) intervention. The FL3X intervention, developed and piloted by our team (R21 DK085483, Mayer-Davis and Seid), uses motivational interviewing to facilitate problem-solving skills training to achieve specific behavioral targets. Further, FL3X employs the methods of adaptive interventions in which patient response to a first-line treatment (FL3X Basic) determines subsequent assignment to a second-line treatment (FL3X Check-In for responders or FL3X Regular for non- responders). FL3X Regular uses additional modular components or 'toolboxes' to further tailor the FL3X intervention to a patient's barriers to adherence related to family communication, diabetes education, social support and diet and physical activity adherence. Our recently completed multisite pilot of FL3X Basic included 61 youth with T1D age 13-16 years, randomized to 31 intervention, 30 control and demonstrated feasibility, acceptability, and promising effects. Attendance at 3 required intervention sessions over a 3-month period was excellent (>90% attendance at each) and 58 of 61 (95%) attended the end-of-study measurement visit. Intervention acceptability was high: 100% of intervention youth said they would recommend FL3X to others. HbA1c decreased by > 0.5% among 41% of intervention participants, compared to 24% of control participants. As for our pilot work, the fully powered FL3X RCT will be coordinated from the University of North Carolina at Chapel Hill, and will include 200 youth, randomized within each of 2 sites that participated in the FL3X Pilot (University of Colorado Denver, Barbara Davis Center for Childhood Diabetes and Cincinnati Children's Hospital Medical Center, Division of Pediatric Endocrinology. Our Specific Aims are: Aim 1: To test the efficacy of the FL3X intervention on the primary outcome (HbA1c), and evaluate secondary outcomes of motivation and problem solving skills, self-management behaviors, risk factors for diabetes complications, and health-related quality of life. Aim 2: To evaluate the cost of intervention delivery and, from the societal perspective, the cost effectiveness of the FL3X intervention. Aim 3: To ensure practicality and immediate applicability of the FL3X intervention.